Grounded Bushings

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6506gary

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I have an electrical inspector that insists we use grounded bushings on all branch circuit conduits entering the service panel if the knockout is a concentric/eccentric opening. Article 100 verifies his request, however it also says there are other ways, I can't seem to find other ways that pertain to this type of installation. Since we are pulling an individual ground wire in each conduit and not using the conduit as a ground, I believe the grounded bushings are not necessary but I can't find anything to support my belief.
 
I'm not thinking art. 100 backs up the inspector cuz it's the definitions. Also, what I find requiring bonding bushings is for services. Nothing about branch circuits. Check art. 250.92 & on. Good luck.
 
Article 100 verifies his request
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How does Article 100 verify his request?

Take a look at 250.97 for the requirements for bonding over 250 volts.

There is no general requirement for a bonding bushing to be installed on the load side of the service disconnecting means for voltages under 250 volts. (There are some requirements in hazardous locations)

Chris
 
6506gary said:
Since we are pulling an individual ground wire in each conduit and not using the conduit as a ground, I believe the grounded bushings are not necessary but I can't find anything to support my belief.
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IMO, the fact that you are doing this takes the KO situation out of the equasion. Ask the AHJ to refer you to an article that backs up his position.
 
Most if not all, load equipment requires a EGC, Section 250.119 gives the allowed types of EGC, including EMT and RMC, and a wire type.
 
m73214 said:
IMO, the fact that you are doing this takes the KO situation out of the equasion. Ask the AHJ to refer you to an article that backs up his position.
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If he's using a metallic raceway I don't believe that to be true. According to Michael Johnston of the IAEI if you are using a metallic raceway the wire type EGC that you install is really secondary protection.

He explained that because of skin effect under a ground fault current a majority of the fault current will travel on the conduit rather that on the wire type EGC. Because of this it is important to always ensure bonding around concentric knockouts.

Now I know the NEC only requires this for circuits over 250v to ground. But I think if the AHJ wants the bushings than you put them in. After all he has the final say. You can cry a river, call his boss and complain all you want. It still won't stop him from holding up your approval until he get's what he wants.

Also, as we all know there are two types of concentric knockouts. Those that when you try to knockout the 1/2" opening the entire knockout falls out. And those that are impossible to knockout and you need to drill the opening out.

Call me stupid, but am I the only one who's ever been told about the skin effect issue in regards to ground faults? If you've heard of it, let me know, I'd hate to think I was wrong all these years but it does make sense to me.

Basically if you install a wire type EGC in a metal raceway. Both are required to be bonded which basically makes them the same conductor. Except that one is inside the other. Skin effect is the tendency for electrons to travel on the surface of a conductor. Since the metal raceway is the outermost surface it makes sense to me that more current would be on the raceway than the conductor.

Again, call me stupid but it makes sense to me.
 
stevee said:
After all he has the final say. You can cry a river, call his boss and complain all you want. It still won't stop him from holding up your approval until he get's what he wants.
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Stevee, welcome to the Forum! We pick our battles here and in the field but compliant is compliant.
 
He explained that because of skin effect under a ground fault current a majority of the fault current will travel on the conduit rather that on the wire type EGC. Because of this it is important to always ensure bonding around concentric knockouts.
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Skin effect is merely the electron flow moving from the center of the conductor to the surface area of the conductor. In the case of an EGC inside a conduit they will act like parallel conductors depending on where the ground fault happens. They both will have skin effect, but it will not make the current flow from the center of the EGC to the surface of the EGC, and then jump through the insulation to the conduit (Bare Cu will allow this if it is in contact with the conduit). The path the ground fault current takes will be the path of least resistance in which most cases will be the EGC (Since panels, j-boxes, etc...are often painted, the conduit is not as good a conductor as Cu, etc..). It really depends on a number of factors (conductivity of the conduit, tempreture, it's resistance where it connects to the rest of the system, etc.., but most importantly where the ground fault occurs).

250.92(B) has the requirement for using grounding bushings and bonding jumpers around KO's. Unfortunately, the way it is worded, it does not specifically reference the service parts that require this type of bonding in 250.92(A)

250.92(B) Method of Bonding at the Service. Electrical continuity at service equipment, service raceways, and service conductor enclosures shall be ensured by one of the following methods:
(4) Other listed devices, such as bonding-type locknuts, bushings, or bushings with bonding jumpers
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The way it is worded, can be read as every conduit requires it, or as common sense would have it, the list in part A requires it. I think you would be fighting a losing battle, but you might be able to point out that branch circuit conduits are not in the list in part A since they are not "service conduits", and therefore are not required by that section. :roll: GL!
 
Sry bout that, didn't see that moderator approval was required for first few posts. Thanks for deleting the duplicate. :grin:
 
Call me stupid, but am I the only one who's ever been told about the skin effect issue in regards to ground faults? If you've heard of it, let me know, I'd hate to think I was wrong all these years but it does make sense to me.
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OK if you insist your stupid, not that I know you or for that matter think you are but you asked for it.

Actually the way it was explained to me is the conduit with an insulated condcutor acts as a choke for the insulated conductor carrying current. NOT sure if this is true or not. But have thought about testing this.

So if I set up a test any ideas on possible minimum lenght of conduit (I am lazy) assume #10 AWG. I'll push 300+ amps. Typical for a 30 amp branch circuit instantaneous fault.
 
How can the conduit act as a choke if the EGC running within it is insulated and there is no direct contact between the two except for the enclousre bond between the ground lug and conduit connector which as the fellow previously stated the fault will take the path of least resistance, which in this case is likely to be the insulated EGC with direct connection to the groung lug. Unless your assuming there's some sort of inductance issue. I do agree that if the EGC is bare, the conduit would create a choke though.
 
6506gary said:
I have an electrical inspector that insists we use grounded bushings on all branch circuit conduits entering the service panel if the knockout is a concentric/eccentric opening. Article 100 verifies his request, however it also says there are other ways, I can't seem to find other ways that pertain to this type of installation. Since we are pulling an individual ground wire in each conduit and not using the conduit as a ground, I believe the grounded bushings are not necessary but I can't find anything to support my belief.
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This may be a bonding issue for the load side of the service. See 250.102(D). Since concentric and eccentric KO's in panelboards are not tested or certified by a laboratory for their current carrying ability and raceways are required to be bonded.
looking at Table 250.122 since sizing is determined by the rating of the OCPD it would seem that all voltages apply.
 
brian john said:
...Actually the way it was explained to me is the conduit with an insulated condcutor acts as a choke for the insulated conductor carrying current. NOT sure if this is true or not. But have thought about testing this....
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Isn't this the reason why we get different available fault currents when comparing with different types of raceway and open air?
 
How can the conduit act as a choke if the EGC running within it is insulated and there is no direct contact between the two except for the enclousre bond between the ground lug and conduit connector which as the fellow previously stated the fault will take the path of least resistance, which in this case is likely to be the insulated EGC with direct connection to the groung lug. Unless your assuming there's some sort of inductance issue. I do agree that if the EGC is bare, the conduit would create a choke though.
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There is no choke effect when the power conductors and EGC are in the same raceway. The choke effect is only when you have a single conductor of an AC circuit installed in a ferrous raceway. The only time we ever have this in a NEC installation is when you install a grounding electrode conductor in a ferrous raceway and this is covered by the rule in 250.64(E).
Don
 
I wish the manufactures would make all equiptment without the knockouts. We would not have to worry about the bond issues as much, and their a pain to knockout. I think they serve their purpose for residential installs, but for comm. and ind. work they suck. I would much rather KO my own holes in disconnects and panels.
 
Just remove all the rings and you can use reducing washers with no need for a grounding bushing.

BTW, I don't know how article 100 supports this inspector either.

Roger
 
I am not sure of this but are Reducing Washers rated for bonding the conduit to the enclosure? I was always taught that the locknut when tightened down properly would "bite" into the enclosure or box. The RW's solution would seem to call for grounding bushings.
 
DAWGS said:
I wish the manufactures would make all equiptment without the knockouts. We would not have to worry about the bond issues as much, and their a pain to knockout. I think they serve their purpose for residential installs, but for comm. and ind. work they suck. I would much rather KO my own holes in disconnects and panels.
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For commercial and industrial work we always order the panels without KO's. The pre-punched ones new seem to be in the correct place.
 
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